1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to a signal driving circuit of a liquid crystal display device and a driving method thereof being arranged.
2. Discussion of the Related Art
A liquid crystal display (LCD) device is widely used to display various images including still images and moving images. The picture quality of an LCD device has greatly improved due to the development of technology for processing fine pixels and to the use of new liquid crystal materials. An LCD device has the characteristics light weight, a slim profile and low power consumption. An LCD device has a wide range of applications that are still broadening. An LCD device is typically composed of a liquid crystal panel, which includes a pair of substrates, one of them being at least made of a transparent glass, and a liquid crystal layer interposed between the two substrates. An LCD device can be classified into two types of devices, a passive matrix-typed LCD device and an active matrix-typed LCD device depending on the structure and driving method of the LCD device.
The passive matrix-typed LCD device has the advantages of easy fabrication and simple driving method, but has the disadvantages of high power consumption with little driving capability and a large number of scan lines. The active matrix-typed LCD device has the advantages of allowing the fabrication of a high quality device since it is structured to have a thin film transistor (TFT) in every pixel within the pixel region such that each pixel can be independently driven. By using a thin film transistor in each pixel, an active matrix-typed LCD device effectively displays moving images.
FIG. 1 is a block diagram of a related art active matrix-typed LCD device. As shown in FIG. 1, the related art active matrix-typed LCD device includes a column driver 3 for supplying the image data, which is input from an external video card 1, to a liquid crystal panel 6. The active matrix-typed LCD device also includes a gamma voltage circuit 4 for supplying signal voltages to the column driver 3, a row driver 5 for supplying scanning signals for controlling the switching operation of the thin film transistors in the liquid crystal panel 6, and a controller 2 for controlling the column driver 3 and the row driver 5. Normally, the liquid crystal panel 6 is of an XGA level (1024×768 pixels) of resolution that includes 1024×3 (RGB) of source lines. Therefore, in an LCD device having a XGA level of resolution, eight column drivers 3 (384×8=3072) are employed, each having an output terminal of 384 channels and four row drivers 5, each having an output terminal of 200 channels.
The analog video data supplied from the digital video card 1, installed in the body of a computer, is supplied to the column driver 3 through the operation of the controller 2. In the alternative, the analog image signal input from a computer is converted into digital video data through an interface module installed in a liquid crystal monitor, and then, is input into an LCD device. The row driver 5 applies one scanning pulse every frame to each scanning line, and the timing of the pulse is normally sequentially applied from the top of the liquid crystal panel 6 to the bottom of the liquid crystal panel 6. The column driver 3 applies liquid crystal driving voltages corresponding to the pixels in one line, while a scanning pulse is applied to the pixels. In other words, the column driver 3 is for applying signal voltages to each signal line.
The thin film transistor connected to the scanning line in the selected pixel is turned “on” when a scanning pulse is applied to the gate electrode of the thin film transistor. Then, the liquid crystal driving voltage passes from the signal line through the drain and the source of the thin film transistor, and is applied to the pixel electrode so as to charge a pixel capacitor. By repeating this operation for each pixel, the image data voltages corresponding to the image signal for each of the pixels for the entire panel are applied in a frame. Further, if the image data voltages are applied to the pixels in only one direction when driving the pixel array, it is necessary to periodically invert the image data voltages applied to the panel to prevent the overheating of the liquid crystal in the pixels due to one-directionally flow of voltage across a substantial portion of the liquid crystal layer for an extended length of time.
The period for changing the direction of the signal voltage, that is, a normal direction to an inverse direction or vice versa, is one field. There are several kinds of methods, such as a field inversion method of changing the voltage polarity of all the pixels in the panel in a field, a line inversion method of alternately changing the voltage polarity of the pixels in a line connected to a scanning line, and a dot inversion method of alternately changing the voltage polarity of each pixel. In all of these cases, the voltage direction should be alternately inverted such that the direction of the pixel voltage (the voltage applied to the pixel electrode from the drain of the thin film transistor) is a normal (+) direction or an inverse (−) direction with respect to the common voltage (Vcom).
FIG. 2 is a detailed block diagram of the column driver depicted in FIG. 1. As shown in FIG. 2, a data latch 41 latches video data 10, 11, 12 input into a pixel. In the case of the LCD device receiving odd number and even number video data, the data latch 41 latches the input video data in the unit of two pixels. A shift register 40 sequentially generates latch enable signals for storing the video data into the line latch in synchronization with external clock signals. The line latch 42 sequentially stores the input video data in synchronization with the latch enable signal. The line latch 42 includes first and second registers (not shown), each having one line size (the number of the source lines connected to one column driver is 384×6 bits in this example). If the video data of one line is stored in the first register, the line latch 42 moves the video data of one line stored in the first register to the second register at the same time. Then, the line latch 42 sequentially stores the video data of another line into the first register.
A digital to analog converter 43 of FIG. 2 receives a plurality of signal voltages from a gamma voltage circuit 4. Then, the digital to analog converter 43 selects at least one or two signal voltages of the plurality of signal voltages input corresponding to each video data from the second register of the line latch. Then, the digital to analog converter 43 divides the selected signal voltage corresponding to the video data, and outputs through each source line of an output buffer 44 as analog image signals. Although not depicted in FIG. 2, a constant common voltage is input into a common electrode in addition to the pixel voltages input to the pixel electrodes through the source lines. The voltage difference between the pixel voltage and the common voltage across the liquid crystal layer determines a gray level of the displayed image of the pixel.
FIG. 3 is a representation of the structure of a digital to analog converter inside the conventional gamma voltage circuit and the column driver. The gamma voltage circuit and the digital to analog converter of FIG. 3 are the same as those in FIG. 2, and like numerals will be used to refer to like elements. As shown in FIG. 3, the digital to analog converter 43 includes a resistance network for distributing the signal voltages 18, which are selected to correspond to the video data 45, into interior gray-scale voltages. The signal voltages 18 can be adjusted from the outside. The gray-scale voltages 47 between each tap point are automatically determined by the resistance network inside the digital to analog converter.
The digital video data 45 input into the column driver (not shown) is input into the digital to analog converter 43 through the data latch and the line latch. A plurality of signal voltages 18 output from the gamma voltage circuit 4 are input into the digital to analog converter 43. The plurality of signal voltages 18 are distributed into a plurality of gray-scale voltages 47 by the resistance network inside the digital to analog converter 43. Each value of the digital video data 45 input as above and the signal voltages 18 supplied by the gamma voltage circuit 4 are distributed into the gray-scale voltages 47 by the resistance network. The distributed gray-scale voltages 47 are output through each signal line, that is, source line as analog image signal through an output buffer 49 corresponding to the video data 45.
The signal voltages 18 output from the gamma voltage circuit 4 are input as positive (+) voltage and negative (−) voltage with respect to the common voltage (Vcom) 50, and are again distributed into a plurality of gray-scale voltages 47 by the resistance network inside the digital to analog converter 43. The gray-scale voltages 47 can be realized differently according to the signal voltages 18 distributed by the external fixed resistance, but are fixed in hardware so that a user cannot change.
The column driver selects one gray-scale voltage 47 of the plurality of gray-scale voltages 47 distributed from the fixed signal voltages 18 supplied by the gamma voltage circuit 4, and corresponding to the input digital video data 45, and then, applies the selected gray-scale voltage to each signal line connected to pixels for liquid crystal cells. The common voltage 50 supplied to the common electrode is individually fixed and applied independently from the gamma voltage circuit 4. However, there is a need to adjust the gray-scale voltage 47 externally of the signal driving circuit such that a user can vary the gradation or the brightness of an LCD device, and nowadays, this need is commercially realized in LCD devices.
FIG. 4 is a graphical representation of the output of gray-scale voltages with respect to a common voltage. As shown in FIG. 4, one gray-scale voltage is arbitrarily selected, and its level of the voltage is illustrated. When a pixel is selected by the row driver, the specific pixel is charged with the one of the gray-scale voltages. When the pixel is selected at the initial time of one horizontal period, the gray-scale voltage is a positive (+) voltage 51 above a common voltage. A negative (−) gray-scale voltage 52 is applied to the selected pixel during the next horizontal period such that its absolute value corresponds to the absolute value of the positive (+) voltage applied to the selected pixel during the previous horizontal period. Therefore, the voltage, which is applied to each pixel, is changed into the gray-scale voltage and alternately changed between the levels of a normal (+) voltage and an inverse (−) voltage. Thus, an alternating current is applied to each pixel. Further, the common voltage (Vcom) 50 can be a direct voltage or an alternating voltage, and the level of each gray-scale voltage is determined with respect to the common voltage 50.
When the absolute values of the normal (+) gray-scale voltage 51 and the inverse (−) gray-scale voltage 52 are different, that is, each level of the gray-scale voltages is not equal to each other with respect to the center of the common voltage 50, the LCD device can be damaged or heated. Further, the characteristics of the pixels can be changed so as to cause a flickering phenomenon or image sticking phenomenon to occur. Therefore, the gray-scale voltages should be maintained symmetric with respect to the center of the common voltage, which is difficult in actual applications. For example, a user needs to adjust the gray-scale by an external control of the common voltage in order to vary the gray-scale or the brightness of an LCD device. However, changing the common voltage causes the absolute values of the normal (+) gray-scale voltage 51 and the inverse (−) gray-scale voltage 52 to be different such that the gray-scale voltages are not symmetrical with respect to the center of the common voltage 50, which causes the problems of image flickering or image sticking.